1. Field of the Invention
The present invention relates to apparatus for the flotation of flocculated solid material in a liquid. The solid material may for example be a sludge which is flocculated and rendered floatable in order to remove it from the liquid, e.g. in the treatment of waste water from industrial processes.
2. Description of the Prior Art
A known form of flotation apparatus (described in more detail below with reference to FIG. 1) has an upward flow passage adjacent a flotation tank. The side wall of this passage on the side towards the tank has an upper end which is curved over towards the tank and ends at a free edge, thus providing a submerged weir over which, in use, the liquid bearing the flocculated solid material ready for floating and collection passes from the upward flow passage into the flotation tank.
In this known apparatus, the solid material is flocculated in a flocculation or coagulation tank, which has a design such that the flocculated material is formed over its whole width and is fed into the upward flow passage across the whole width of this passage. To float the flocculated components, the flocculated material is carried up the upward flow passage to the surface of the liquid by small bubbles of gas or air. At the surface the flocculated particles should form a continuous layer which spreads out over the flotation tank. The liquid itself passes slowly through the flotation tank and can escape at the bottom thereof. The floating solids are removed by a skimming apparatus.
This general process of flocculation, flotation and skimming is well known and need not be described in this specification which is concerned with an improvement in one feature of the flotation step.
It should be noted that, to achieve good separation of solids and the liquid, it is important that the flocculated material is able to form as complete a layer as possible on the liquid surface and that this layer is affected as little as possible by the subsequent flow of the liquid in the flotation tank.
In this known apparatus, the vertical cross-section of the flotation tank is substantially rectangular and the upward flow passage and flotation tank are separated by a nearly vertical partition which directly separates the passage and the tank and at the top of which is the weir described above. Considerable turbulence in the flotation tank behind the overflow edge has been found unavoidable; as a result of this turbulence, a substantial fraction of the flocculated flotation material becomes mixed again with the liquid again and settles in the flotation tank. This fraction is then out of reach of the skimming mechanism, and contaminates the flotation tank. Also the effluent from the apparatus is significantly dirtier, because of this fraction.
DE No. 2 641 718 shows a flotation tank arrangement in which the side wall of the upward flow passage is straight, and is joined below its upper edge by a sloping side wall of the flotation tank. The performance of this arrangement is not known.